1. Field of the Invention
The present invention relates to an electrographic imaging system using an electrostatic recording head.
2. Description of the Prior Arts
FIG. 1 is a schematic view for showing a fundamental structure of a printing part in an electrographic imaging apparatus using a known single surface control electrostatic recording head (hereinafter referring to as a recording head). In FIG. 1, the reference numeral (1) designates recording stylus (only one stylus is shown) and many recording needles are orthogonally arranged to a paper. A plurality of groups of control electrodes (2) (only one group is shown) are arranged for each predetermined number of the recording styluses (1) at both sides of the recording styluses. An electrostatic record medium (6) made of a conductive substrate (4) and a dielectric layer (5) is press-contacted with a head surface of a recording head (3) having said structure.
Under such condition, a negative recording stylus voltage v.sub.n is uaually applied to the recording stylus (1) and a positive control voltage v.sub.c is simultaneously applied to the control electrode (2) whereby an electrostatic latent image is formed on the surface of the dielectric layer of the electrostatic record medium (6). The latent image formed on the electrostatic record medium (6) is developed in a toner development by a developing device (8) and the toner is melt-bonded by a heat-fixing device (9).
FIG. 2 shows waveforms v.sub.n and v.sub.c in the conventional electrographic imaging system; and v.sub.p is a potential of the conductive medium (4) just below the control electrode (2) to which the control voltage v.sub.c is applied.
FIG. 3 is an approximate equivalent circuit for the electrostatic record medium to which the control voltage v.sub.c is applied; and C.sub.s1 designates and electrostatic capacity of the dielectric layer (5) just below the control electrode (2) to which the control voltage v.sub.c is applied; C.sub.s2 designates an electrostatic capacity of the dielectric layer (5) at the part contacted with the control electrode being adjacent to the control electrode to which the control voltage v.sub.c is applied; R.sub.g designates a resistance of the conductive medium (4) from the earth point to the control electrode to which the control voltage v.sub.c is applied; R.sub.s designates a resistance of the conductive medium (4) between said two control electrodes.
The potential v.sub.p has the waveform shown by the curve a in FIG. 2. That is, a differential waveform of v.sub.c having a time constant of ##EQU1## is substantially equal to the waveform of v.sub.p.
The resistances R.sub.g and R.sub.s are remarkably decreased depending upon the elevation of the temperature or humidity. Accordingly, v.sub.p is in peak value at the leading edge of the control voltage v.sub.c and remarkably attenuates as shown by the curve b in FIG. 2 at high temperature and high humidity. The latent image is formed on the dielectric layer (5) by a voltage of the sum of absolute values of v.sub.p and the recording stylus voltage v.sub.n.
In the case of a many stylus electrode type recording, recording multi-styluses are grouped and arranged in parallel. Accordingly, an electrostatic capacity between the recording styluses is ranging from about 150 to 400 pF. Thus, the time constant .tau..sub.n at the leading edge of the recording stylus voltage v.sub.n is longer than the time constant .tau..sub.c at the leading edge of the control voltage v.sub.c. In the condition of high temperature and high humidity for characteristics shown in the curve b in FIG. 2, the potential v.sub.p attenuates to be small before reaching v.sub.n to the saturated value whereby the record density is remarkably decreased to cause defect of the recording to be disadvantageous.
The decrease of the record density at high temperature and in high humidity is the first disadvantage which can not be avoided in the conventional system.
Moreover, in the conventional system, a ghost image caused by a leakage of voltage to the adjacent control electrode is found as the second disadvantage.
FIG. 4 is a fundamental circuit diagram used in the conventional system. The reference numeral (101) designates recording multi-styluses electrodes which are linearly arranged; (201) designates control electrodes which are divided into units having predetermined electrodes and are arranged as groups at both sides of the recording styluses(101). In the embodiment, the control electrodes are divided into each unit for each five recording styluses (101).
The recording styluses(101) are connected to form alternately groups A and groups B for each five recording styluses so as to form a total 2n blocks.
In said structure, when the recording of the A.sub.1 block is carried out, the recording stylus voltage v.sub.n is applied from the group A recording signal source (10a) depending upon the predetermined recording pattern for A.sub.1 and simultaneously, the control voltage v.sub.c is applied from the control voltage source (11) to S.sub.1 and S.sub.2 of the control electrodes (201). The recording is carried out only in the block A.sub.1 to which both of v.sub.n and v.sub.c are simultaneously applied.
When A.sub.k is recorded, the control voltage v.sub.c is applied to S.sub.2k-1 and S.sub.2k. When B.sub.k is recorded, the control voltage v.sub.c is applied to S.sub.2k and S.sub.2k+1. The recording for one line is sequentially completed.
When the state applying the pulse voltage v.sub.c to S.sub.2k-1 and S.sub.2k is considered, the potential v.sub.g of the electrostatic record medium just below the adjacent control electrodes S.sub.2k-2 and S.sub.2k+1, is elevated to cause ghost voltage v.sub.g because of the leakage of v.sub.c as shown by the curves a and b in FIG. 5.
The curves a and b respectively correspond to v.sub.g in the conditions of low humidity and high humidity. Thus, the reason why v.sub.g is suddenly elevated in high humidity, is to decrease the resistance r.sub.s of the conductive medium of the electrostatic record medium in the equivalent circuit shown in FIG. 3. The time constant for charging to the capacity C.sub.s2 from the adjacent control electrode is shortened so as to elevate v.sub.g. Thus, the recording should be performed only in the block A.sub.k, nevertheless, the recording is also performed in the block A.sub.k+1 and A.sub.k-1. The excess recorded images are referred to as the ghost. The formation of the ghost is the second disadvantage in the conventional system.